Self-lofting of wildfire smoke in the troposphere and stratosphere: simulations and space lidar observations

Abstract

Abstract. Wildfire smoke is known as a highly absorptive aerosol type in the shortwave wavelength range. The absorption of sunlight by optically thick smoke layers results in heating of the ambient air. This heating is translated into self-lofting of the smoke up to more than 1 km in altitude per day. This study aims for a detailed analysis of tropospheric and stratospheric smoke lofting rates based on simulations and observations. The main goal is to demonstrate that radiative heating of intense smoke plumes is capable of lofting them from the lower and middle free troposphere (injection heights) up to the tropopause without the need of pyrocumulonimbus (pyroCb) convection. The further subsequent ascent within the lower stratosphere (caused by self-lofting) is already well documented in the literature. Simulations of absorbed solar radiation by smoke particles and resulting heating rates, which are then converted into lofting rates, are conducted by using the ECRAD (European Centre for Medium-Range Weather Forecasts Radiation) scheme. As input parameters thermodynamic profiles from CAMS (Copernicus Atmosphere Monitoring Service) reanalysis data, aerosol profiles from ground-based lidar observations, radiosonde potential temperature profiles, CALIOP (Cloud–Aerosol Lidar with Orthogonal Polarization) aerosol measurements, and MODIS (Moderate Resolution Imaging Spectroradiometer) aerosol optical depth retrievals were used. The sensitivity analysis revealed that the lofting rate strongly depends on aerosol optical thickness (AOT), layer depth, layer height, and black carbon (BC) fraction. We also looked at the influence of different meteorological parameters such as cloudiness, relative humidity, and potential temperature gradient. To demonstrate the applicability of our self-lofting model, we compared our simulations with the lofting processes in the stratosphere observed with CALIOP after major pyroCb events (Canadian fires in 2017, Australian fires in 2019–2020). We analyzed long-term CALIOP observations of smoke layers and plumes evolving in the UTLS (upper troposphere and lower stratosphere) height region over Siberia and the adjacent Arctic Ocean during the summer season of 2019. Our results indicate that self-lofting contributed to the vertical transport of smoke. We hypothesize that the formation of a near-tropopause aerosol layer, observed with CALIOP, was the result of self-lofting processes because this is in line with the simulations. Furthermore, Raman-lidar-based aerosol typing (in Leipzig and the High Arctic) clearly indicated the dominance of smoke in the UTLS aerosol layer since August 2019, most probably also the result of smoke self-lofting.

Annual cycle of aerosol properties over the central Arctic during MOSAiC 2019–2020 — light-extinction, CCN, and INP levels from the boundary layer to the tropopause

“…This unique event was discussed in detail by Ohneiser et al (2021). The smoke originated from record-breaking wildfires in central and eastern Siberia, north of Lake Baikal, in the summer of 2019 (Johnson et al, 2021;Ohneiser et al, 2021Ohneiser et al, , 2023Xian et al, 2022b;Sorenson et al, 2022) and affected the ozone layer (Ohneiser et al, 2021;Voosen, 2021;Ansmann et al, 2022). The smoke reached the UTLS height range most probably by self-lofting processes (Ohneiser et al, 2021(Ohneiser et al, , 2023.…”

“…The ice nucleation efficiency of aged smoke particles is determined by organic material (organic carbon, OC). The black carbon (BC) or soot content is typically 2-3% (Dahlkötter et al, 2014;Yu et al, 2019;Torres et al, 2020;Ohneiser et al, 2023) and has no relevant impact on the ice-nucleating efficiency of aged wildfire smoke particles. Biomass-burning particles also contain humic like substances (HULIS) which represent large macromolecules that could serve as INP at low temperatures of −50 to −70°C (Wang and Knopf, 2011;Wang et al, 2012;Knopf et al, 2018).…”

“…The smoke originated from record-breaking wildfires in central and eastern Siberia, north of Lake Baikal, in the summer of 2019 (Johnson et al, 2021;Ohneiser et al, 2021Ohneiser et al, , 2023Xian et al, 2022b;Sorenson et al, 2022) and affected the ozone layer (Ohneiser et al, 2021;Voosen, 2021;Ansmann et al, 2022). The smoke reached the UTLS height range most probably by self-lofting processes (Ohneiser et al, 2021(Ohneiser et al, , 2023. Large amounts of Siberian smoke were transported into the central Arctic at all heights from mid-June to mid-August 2019 and caused extremely large 500-550 nm AOT values exceeding daily mean values of 0.2 in the polar region from 70°to 90°N from 24 July to 22 August 2019 (Xian et al, 2022b).…”

“…A first case study was presented in Engelmann et al (2021). Aged wildfire smoke in the upper troposphere and stratosphere consists to more than 95% of organic material (Yu et al, 2019;Kablick et al, 2020;Khaykin et al, 2020;Torres et al, 2020;Ohneiser et al, 2023). Only 2-3% is black carbon.…”

Assessing the Impact of Self‐Lofting on Increasing the Altitude of Black Carbon in a Global Climate Model

“…Among other studies, self-lofting has been considered in the contexts of individual smoke plumes from large fires (e.g., Herring & Hobbs, 1994;Radke et al, 1990), distinct layers of absorbing haze in the free troposphere (e.g., Haywood et al, 2021), the impacts of absorbing aerosol in volcanic plumes (e.g., Muser et al, 2020), and as a consequence of nuclear conflict (Pausata et al, 2016). As outlined by Boers et al (2010) and Ohneiser et al (2023), the gain in elevation will depend on a range of factors, including the strength of aerosol absorption, the stability of the atmosphere and the length of time before the aerosol is dispersed or removed. In a relatively extreme example, Boers et al (2010) showed self-lofting could raise an optically thick and highly absorbing aerosol layer to the tropopause in 3-4 days Moorthy et al (2016) estimated slightly more modest ascent rates of 1 km per day for absorbing haze in the mid-troposphere above India.…”